Picosecond time-resolved measurements of dense plasma line shifts

Picosecond time-resolved x-ray spectroscopy is used to measure the spectral line shift of the $1s2p\text{−}1s^2$ transition in He-like Al ions as a function of the instantaneous plasma conditions. The plasma temperature and density are inferred from the Al $\mathrm{H}{\mathrm{e}}_{\alpha }$ complex using a nonlocal-thermodynamic-equilibrium atomic physics model. The experimental spectra show a linearly increasing redshift for electron densities of $1-5\times {10}^{23}\mathrm{c}{\mathrm{m}}^{-3}$. The measured line shifts are broadly consistent with a generalized analytic line-shift model based on calculations of a self-consistent field ion-sphere model.

Figure 1

(a) Experimental setup. (b) Example streaked $\mathrm{H}{\mathrm{e}}_{\alpha }$ emission from a rapidly heated Al surface layer. The Li-like satellites, intercombination line, and $\mathrm{H}{\mathrm{e}}_{\alpha }$ line are shown.

Figure 2

(a) X-ray spectra from an Al surface layer at $t_0+12\mathrm{ps}$ (blue) and $t_0+275\mathrm{ps}$ (green). (b) The measured peak positions were identified with tabulated transition energies using an analytic dispersion calibration derived from Bragg's law for the spectrometer geometry.

Figure 3

(a) Streaked $\mathrm{H}{\mathrm{e}}_{\alpha }$ emission from a buried Al layer. (b) The plasma conditions are inferred from the $\mathrm{H}{\mathrm{e}}_{\alpha }$ intensity ratio (red) and FWHM (blue). (c) The synthetic spectrum corresponding to $T_e=330\mathrm{eV}$ and $\rho =0.9\mathrm{g}/\mathrm{c}{\mathrm{m}}^3$ (red) is compared to the data (blue).

Figure 4

(a) Comparison of the data (black) to the analytic line-shift model (blue) and apparent shifts (red). (b) Comparison of the data (black) to the analytic line-shift model plus apparent shifts (orange) and the line-shift model with a scaled ion-sphere radius (purple). The optimum scale factor was determined by a single-parameter least-squares fit to the data.